(a) Field of the Invention
The present invention relates to a method for detecting an end point of etching in a plasma-enhanced etching process and, more particularly, to a technique for detecting an end point of etching in a gas-plasma-enhanced etching process by using a spectrum analyzing technique.
(b) Description of the Related Art
A gas-plasma-enhanced etching technique is known as one of dry etching techniques used in fabrication of semiconductor devices. FIG. 1 shows a typical plasma-enhanced etching system having a pair of parallel planar electrodes.
The parallel planar electrodes include a cathode 12 connected to a high-frequency power source 14, and an anode 13 connected to the ground, both of which are disposed in a reaction chamber 11. A semiconductor wafer 15 is mounted on the surface of the cathode 12. Etching gas is introduced through a gas inlet port 16 into the reaction chamber 11, and subjected to plasma-excitation by a high-frequency power between the cathode 12 and the anode 13 to generate gas plasma 17. The gas plasma 17 thus generated etches the surface of the semiconductor wafer 15 in a dry etching process. The reacted gas generated in the reaction chamber 11 by chemical reaction is exhausted through a gas outlet port 18 from the reaction chamber 11.
The reaction chamber 11 has a quartz window 19 for monitoring the light intensity of plasma with respect to elapsed time, which is subjected to a variety of data processing such as calculation of a second derivative (second order differential) of the light intensity.
An end point of the etching is generally determined based on the measurement of the light intensity by using a spectrum analyzing technique and the data processing thereof. Patent Publication JP-A59-94423 describes a technique for detecting the end point of the etching, wherein the end point is determined based on a second derivative of the light intensity of the plasma in the reaction chamber calculated with respect to time. More specifically, the end point is detected at the timing when the second derivative assumes zero after exceeding a threshold in the absolute value thereof.
FIG. 2 shows two different curves (curves xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d) of the light intensity generated by the plasma-enhanced etching, whereas FIGS. 3A and 3B show second derivative curves for the light intensity curves xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d, respectively, shown in FIG. 2. The two light intensity curves xe2x80x9cAxe2x80x9d and xe2x80x9cBxe2x80x9d are obtained, for example, by etching an aluminum film on a semiconductor wafer to form interconnect patterns for about one minute.
The curve xe2x80x9cAxe2x80x9d in FIG. 2 is obtained in the case of etching a larger area ratio, such as 60% of the etching area to the total area, whereas the curve xe2x80x9cBxe2x80x9d is obtained in the case of etching a smaller area ratio, such as 20% of the etching area to the total area.
As shown in FIG. 2, if the etching area ratio is large, a larger etching time is needed, with the light intensity being maintained at a high level during the etching and the reduction of the light intensity being steep at the end point of the etching. On the other hand, if the etching area ratio is small, a smaller etching time is needed, with the light intensity being maintained at a lower level during the etching and the reduction of the light intensity being moderate at the end point of the etching.
As a result, the second derivative, as shown in FIG. 3A, for the curve xe2x80x9cAxe2x80x9d eventually exceeds a threshold xe2x80x9caxe2x80x9d in the absolute value thereof for effectively informing the end point of the etching, whereas the second derivative, as shown in FIG. 3B, for the curve xe2x80x9cBxe2x80x9d does not exceed the threshold xe2x80x9caxe2x80x9d in the absolute value thereof and thus does not effectively informing the end point.
In the dry etching process using the plasma-enhanced etching technique in general, the control of the etching speed, in-plane uniformity of the etching and the dimensions or shape of the profile in the etched film has become more and more difficult to achieve. This is partly because of the difference in the etching area together with the smaller process margin in the fabrication steps. The smaller process margin is more and more noticeable along with the smaller design rule and higher performance required for the semiconductor devices.
In the above situations, it is difficult to effectively detect the end point of the etching if, for example, different products having different etching areas are etched in a single etching step. This reduces the yield of the products, or the ratio of the number of good products to the number of the total products, and raises the costs for the products. Thus, it is desired to effectively detect the end point of the etching in the plasma-enhanced etching process.
In view of the above, it is an object of the present invention to provide a method for detecting an end point of etching in a plasma-enhanced etching process, which is capable of effectively detecting the end point of the etching.
The present invention provides a method for detecting an end point of etching in a plasma-enhanced etching of a film, comprising the steps of measuring a light intensity of plasma with respect to time, determining whether or not an etching area ratio of the etching step is above a threshold based on a curve of the measured light intensity, selecting a sampling time interval based on the etching area ratio, calculating a second derivative of the light intensity by using the selected sampling time interval, and detecting an end point of the etching based on the second derivative of the light intensity.
In accordance with the method of the present invention, the second derivative of the measured light intensity exceeds the threshold in the absolute value thereof due to the selection of the sampling time interval, irrespective of the level of the light intensity itself. Thus, the time instant at which the second derivative assumes zero after exceeding the threshold can be effectively detected which indicates the end point of the etching in the plasma-enhanced etching.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.